Hydrophilic sulfonated monomers including styrene sulfonic acid sodium salt (SSA) and 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) were grafted onto a brominated silica nanoparticle (BSN) as a macroinitiator via surface-initiated atom transfer radical polymerization (SI-ATRP) at pH of about 10 in the presence of CuBr/bipyridine as a catalyst/ligand system and water/methanol (3/1 v/v) mixture as a solvent at 25 degrees C for 24 h. To synthesize BSN macroinitiator, silica nanoparticle (SN) was fist modified with 3-aminopropyltriethoxy silane and then brominated via reacting with 2-bromopropionyl bromide in the triethylamine/THF solution. Modified SNs were characterized by FTIR spectroscopy and thermogravimetric analysis (TGA). Grafting percentage of the monomers onto the SNs was calculated from TGA thermograms to be 30.6%, 47.6% (for the initial AMPS concentration of 0.8 and 2.0 M, respectively) for PAMPS and 28.1% (for the initial SSA concentration of 1.2 M) for PSSA. The PVA-based nanocomposite membranes containing various amounts of the pristine and modified SNs were prepared by solution casting and then cross-linked at 40 degrees C for 15 min in an acetone solution containing 0.5 M glutaraldehyde and 0.12 vol% HCl. SEM photograph and X-ray mapping of the cross-sectional view of the membranes showed that pristine and modified SNs have homogeneously dispersed at a nanometric scale in the PVA matrix. It was found that ion exchange capacity, water uptake (WU) and proton conductivity of the PVA-based membranes increase by increasing the SN, especially sulfonated polymer-modified SN, loading up to 10 wt%. It was found that proton conductivity of the nanocomposite membranes increases by increasing the grafting percentage of the sulfonated monomer onto SNs. Moreover, for similar grafting,percentage of the monomers onto SNs, higher WU and proton conductivity was observed for a membrane containing SN modified with the PAMPS chains. Results showed that both hydrophilic nature and grafting efficiency of the sulfonated hydrophilic monomers can play an important role in the proton conductivity of the PVA-based membranes. Tensile modulus and tensile strength of the PVA-based membranes enhanced by adding modified SNs to the PVA matrix. Methanol uptake and permeability of the PVA-based polymer electrolyte membranes were also decreased by adding SNs to the PVA matrix. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.